Hitherto, as photosetting resins, there have been used monomer compositions added with a small quantity of a photopolymerization initiator, said monomer compositions comprising a polyfunctional monomer such as 1,6-hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, dicyclopentanyl di(meth)acrylate, tri((meth)acryloxymethyl) isocyanurate, urethane-modified di(meth)acrylate, epoxy-modified di(meth)acrylate, trimethylolpropane tri(meth)acrylate and the like, and further, if necessary, with a monofunctional monomer such as methyl (meth)acrylate, ethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, hydroxyethyl (meth)acrylate, N-vinylpyrrolidone and the like.
There are also used colored photosetting resins prepared by adding a dye or pigment to said photosetting resins and ones added with an organic or inorganic filler to regulate the color tone or mechanical properties of the set or cured resins.
These photosetting resins can be set by irradiation of light such as ultraviolet light for a short time, from several seconds to several minutes, to give a desired set or cured resin. Further, most of these photo-setting resins are one-pack type and very easy to handle. They also have a characteristic that the thickness where setting takes place is usually within a range from several .mu.m to several hundred .mu.m, so that they find a wide scope of application including, for instance, coating material, printing ink and adhesive.
Recently, there has been developed a photosetting resin as a material for dental use, which can be set with visible light. In the case of this photosetting resin for dental use, since visible light is used for setting the resin, there can be relatively easily obtained a set or cured resin even when the photosetting resin has a large thickness of up to 5-6 mm or is an opaque one mixed with a filler. Accordingly, this type of photosetting resin is applied advantageously as a filling resin for cavities or hard resin to be used as resin veneer for cast crown, and for other dental applications.
A need is arising for further expanding the scope of application of the photosetting resins by making the most of their advantages such as mentioned above. Japanese Patent Application Laying Open (KOKAI) No.58-179212, for instance, discloses a photosetting resin applied as a material for producing resin molded products.
However, in application of these conventional photosetting resins to formation of the resin molded products, the obtained molded products were indeed excellent in surface hardness and other properties such as tensile and compressive strength, but generally the molded products would become hard and fragile so that only a slight flaw to the molded product could result in break thereof.
For instance, in case a photosetting resin having said dicyclopentanyl diacrylate as a monomer component is applied to a hard coating material, there can be obtained a coating film having excellent surface hardness. But in case said photo-setting resin is made into a plate-shaped molded article having a thickness of about 2-3 mm by using a frame, there is merely obtained a frangible molded article which is easily broken when given only a slight deformation.
Such defects of the conventional photosetting resins are due to the very small breaking strain of the set or cured resins as noticed in a bending test or tensile test of said resins.
Such problem of fragility of the conventional photosetting resins can be overcome by enlarging the breaking strain of the set or cured ones.
But if a monomer having soft properties is simply used as a constituent of a photosetting resin in a bid for attaining said object, although the breaking strain will be enlarged, there will also arise another problem: sharp reduction of mechanical strength of the set resin, making it impossible to obtain a molded article having well satisfactory mechanical properties.
Thus, the conventional photosetting resins were frangible and poor in toughness and therefore unsuited for use as a material for forming strong and tough resin molded articles.